Investigators believe Aaron Alexis, a former Navy reservist, was the lone gunman in the shooting spree at the Navy Yard in Washington D.C. earlier today in which 12 people were killed before the suspect was killed in a firefight with police, and authorities have lifted a shelter in place for the remaining residents in the area.

The 12 deceased shooting victims range in age from 46 to 73 years old, Washington D.C. Mayor Vincent Gray said at an evening news conference. Officials are still notifying the families of about half of the 12 people who were killed, he said.

"We have no evidence that any active military are amongst the victims," Metropolitan Police Chief Cathy Lanier said.

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Aaron Alexis: The Man Behind the Washington Navy Yard Shooting

Officials said three others suffered gunshot wounds and five more people suffered other injuries.

The death of Alexis, 34, of Fort Worth, Texas, brought the toll of the carnage at the Navy Sea Systems Command headquarters to 13. The shooter's identity was confirmed based on a partial finger print analysis, authorities said.

The suspect had a security clearance that allowed him onto the Navy Yard as part of his civilian subcontracting work, officials said.

Courtesy FBI

Aaron Alexis, deceased, is believed to be responsible for the shootings at the Washington Navy Yard, in the Southeast area of Washington, DC, around 8:20 a.m. on Sept. 16, 2013.

Alexis and members of the subcontracting team, according to law enforcement sources, were staying at a Residence Inn about a mile from the Navy Yard. The suspect's car was found on the Washington Navy Yard, law enforcement sources said.

A senior law enforcement official said he used his security clearance to get on campus and it appeared he did not force his way onto the property. Officials were in the process of getting a search warrant to search the vehicle, which was described as a rental car.

Authorities had earlier said they were searching for a possible second suspect, but officials said at the evening news conference said the search for a black suspect in olive-drab uniform age 40 to 50 has been exhausted.

Earlier today, Lanier praised the works by police officers who responded to the incident.

"I think the actions by the police officers, without question, helped to reduce the number of lives lost," she said.

Lanier said police and the suspect exchanged gunfire "multiple times" before he was shot and killed in a final gunbattle.

Valerie Parlave, assistant director in charge of the FBI's Washington D.C. field office, said investigators were trying to learn everything they can about Alexis.

"No piece of information is too small," Parlave said.

She asked anyone with information about his recent movements, contacts and associates to contact the FBI at 1-800-CALL-FBI.

Another man who was sought for a possible connection to the shooting was located this afternoon and was no longer a "suspect or person of interest," according to a tweet from the Metropolitan Police Department.

Lanier declined to discuss what evidence led police to believe the massacre could have potentially been carried out by more than one person.

"We have reason to believe these people may be involved and we want to talk to them," Lanier said at an afternoon news conference. She said there was "no known motive" for the massacre.

Among the wounded was a law enforcement officer who was shot in an exchange with the gunman.

The shooting brought parts of Washington D.C. to a standstill.

The Washington Nationals, whose stadium is a couple blocks away from the Naval Sea Systems Command headquarters, said the team's game tonight against the Atlanta Braves was postponed until Tuesday.

The Senate complex was placed under a temporary lockdown this afternoon "in light of the uncertainty surrounding the shooting" and the possibliity that a second shooter might have been at large, Senate Sergeant at Arms Terrance W. Gainer wrote in a note to the Senate community.

At nearby Reagan National Airport this morning, a ground stop was imposed by the FAA. All planes have since resumed flying out of the airport, a Metropolitan Washington Airports Authority spokeswoman told ABC News.

Nine D.C. public schools were placed on lockdowns this morning, according to the District of Columbia Public Schools' Twitter account, but all schools were dismissed as scheduled this afternoon.

Washington D.C. Rep. Eleanor Holmes Norton told reporters the district has not "had a day like this since 9/11."

President Obama ordered the flags at all federal and military installations to be flown at half-staff through Friday in honor of the victims.

At the beginning of a news conference today, Obama said he was briefed on the shooting.

"We are confronting yet another mass shooting, and today it happened on a military installation in our nation's capital," he said.

Obama said he wanted a "seamless" investigation into the shooting and was standing with the victims and their families affected by what he called a "cowardly act."

"It targeted our military and civilian personnel, men and women who were going to work, doing their job protecting all of us. They are patriots and they know the dangers of serving abroad," Obama said. "But today they faced the unimaginable violence that they won't have expected here at home."

Two Officers Among the Injured

Two law enforcement officers were among the injured when the suspect fired shots inside the 3,000-person building at 8:20 a.m. Law enforcement officials initially told personnel to evacuate the building but they were later told to shelter in place.

A spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives sent a team of special agents to help secure the scene. This is the same team that helped apprehend Boston Marathon bombing suspect Dzhokhar Tsarnaev, according to the spokesman.

Three gunshot victims were taken to Washington Medical Center with "severe injuries," according to Janis Orlowski, chief medical officer at the hospital.

The victims were described as a male Metropolitan police officer who had "multiple gunshot wounds to his legs," a woman who was shot in the head and the hand and another woman who was shot in the shoulder.

Orlowski said all three were in critical condition, however they were conscious and were expected to survive.

Navy Yard Witnesses Heard Series of Loud Pops

Patricia Ward, a logistics management specialist at the Navy Yard, said she heard a series of shots, at least seven, in rapid succession.

"A few of us just ran outside the side exit," Ward said.

Outside the building she saw a security guard with her gun drawn who told them to run and shelter.

Ward said the building has security.

"You need a card to enter the building. It's very hard to get in without a card," she said.

"Being with the incident that happened today, not secure enough for me," Ward said.

Frank Putzo, an attorney at the Navy Yard, told ABC News affiliate WJLA-TV he was on the fourth floor of the building when the shooting began.

"We heard three sounds, it sounded like a table collapsing on the ground," he said.

He said he came out of his office and heard a "very loud pop" about a minute later that he estimated was about 100 feet away.

"When that happened everyone said, 'This is no drill, go, go, go,'" he said. "And a whole bunch of us were able to make it to the emergency exits. And we heard several more shots."

The Naval Sea Systems Command, the largest of the Navy's five commands, is responsible for engineering, building, buying and maintaining ships, submarines and combat systems in the Navy's fleet.